Drag-painting unexpectedly drops 3D cube items

[deathcap]

In the demo http://deathcap.github.io/inventory-window/, try picking up an item from the 1st window, then holding the right mouse button down to drop the items over the 3rd window, one at a time.

This works well with the stick item since it has a flat texture, but not so well with the grass block (3D CSS cube, uses https://github.com/deathcap/cube-icon). Multiple items are dropped onto the same slot as you drag over the same slot:

it shouldn't drop until the cursor hovers over a different slot, as seen with the stick:

This bug makes it cumbersome to craft with blocks (e.g., in voxel-workbench)